In an emergency, such as, for example, if faults occur in the electronics, the sensors or the actuators, adjustment devices or camshaft adjusters of this kind must bring the camshaft into a defined emergency running position (e.g., fail-safe position) to ensure that the internal combustion engine can continue to operate at least with limitations. In particular, the emergency running position can be chosen so that the internal combustion engine can also be started in this position.
Usually, i.e., in normal operation of the internal combustion engine, the camshaft is moved under control to its basic or emergency running position (for example, with the help of an adjustment motor) when the internal combustion engine is switched off, i.e., as a rule the “late” position in the case of an inlet camshaft or the “early” position in the case of an exhaust camshaft. If, however, the internal combustion engine stalls or the electronics (e.g., the adjustment motor) fails, the camshaft can be in an undefined position away from the emergency running position, which can lead to problems when the internal combustion engine is restarted.
Devices for adjusting the camshaft of an internal combustion engine are normally in the form of an adjustment gearbox, in particular a three-shaft gearbox, which, for example, has an input drive shaft connected to the crankshaft, an output drive shaft connected to the camshaft and an adjustment shaft connected to an electric adjustment motor.
An electromechanical phase adjuster for adjusting the phase position of a camshaft relative to a crankshaft of an internal combustion engine, with which the range of possible phase positions (adjustment angle range) of the camshaft is limited by two end stops, the so-called late end stop and the so-called early end stop, is disclosed in document WO 2005/008034.
In an embodiment, these two end stops are connected to a rotating disc of the gearbox which is connected to the camshaft gear and interact with a stop which is located on the gearbox output side on a rotating disc of the gearbox which is connected to the camshaft.
The gearbox of the electromechanical phase adjuster is not self-locking, i.e., a torque at its gearbox input side causes a rotation at its gearbox output side and vice versa.
Furthermore, the adjustment gearbox has a negative rotational transmission ratio, i.e., the direction of rotation at the gearbox input side associated with the adjustment drive is opposite to that at the gearbox output side. As a result of this, a retardation of the gearbox input side effects an adjustment of the gearbox output side in the “early” direction. If this fails, then a locking device causes the camshaft to move into a specified emergency running position. Depending on the design, the stop which is arranged on the camshaft or the two end stops which limit the adjustment angle range is/are designed to be movable for this purpose. For the purpose of repositioning, a pre-tensioned spring, which in normal operation is held in its pre-tensioned position by means of a latching mechanism, is provided on the stop to be moved. As a result of activating an unlocking mechanism of the locking device, the latching mechanism is released so that, due to its pre-tensioning, the spring can move the associated stop.
A blocking mechanism, which is likewise associated with the stop to be moved, prevents it moving in a direction which opposes the spring pre-tensioning. In an emergency situation in which the electric adjustment drive has failed, the unlocking mechanism, therefore, unlocks the latching mechanism so that the stop of the camshaft is brought into contact with the end stop of the input drive gear. The blocking mechanism prevents the possibility of the camshaft moving in opposition to the end stop which is in contact therewith.
When the phase adjustment direction is subsequently changed again, the first end stop or the stop which rests against the end stop is moved towards its emergency running position until it reaches this position. In the emergency running position, the second end stop also rests against the stop, and thus, locks the phase adjustment device in its emergency running position.
In accordance with this embodiment, a freely definable mid-position (position within the adjustment range) can be chosen as the emergency running position (fail-safe position). It is not disclosed, however, how the actuators used can leave the emergency running operating state (fail-safe mode) in order to change over to normal operation. This is, however, particularly important when, for example, it transpires that the emergency running mode has only been initiated as a result of a plausibility problem or a temporary fault.
Furthermore, a device for releasably connecting and adjusting the camshaft and the crankshaft of an internal combustion engine is disclosed in WO 03/095803 A1. Here, the emergency running position of the camshaft is achieved by rotating the drive shaft, which is connected to the crankshaft, and by a suitable stationary gear ratio. In this embodiment, the emergency running position is therefore achieved in that contact is made with a mechanical end stop. The emergency running position therefore corresponds to the maximum early or late setting on the respective early end stop or late end stop of the camshaft.
It can, however, transpire that the camshaft does not achieve its final emergency running position, especially when the adjustment motor produces a braking torque when it ails or when an actuating electronic device fails. Furthermore, a reliable positioning of the camshaft is not guaranteed, thus enabling it to leave its emergency running position, possibly in an uncontrolled manner. This can occur particularly as a result of the inertia-induced torques through the adjustment motor which the internal combustion engine produces at the gearbox input of the adjustment gearbox when accelerating or braking, or when strongly alternating torques act on the camshaft.
Furthermore, DE 10 2004 061 710 A1 discloses a camshaft adjustment device which, in an emergency, can be locked with the help of a locking element for locking a component fixed to the camshaft to a component fixed to the crankshaft.
Furthermore, an electric camshaft adjuster, in which a pre-tensioned spring pushes the camshaft adjuster back into an appropriate end position (maximum early position or maximum late position of the camshaft), is disclosed in WO 2011/104051 A1. The disadvantage of such an embodiment can be that the spring is not only active in emergency mode, but also positively acts on the camshaft over the whole adjustment range thereof, which may have a negative effect with regard to its energy consumption. If the spring force of this adjustment spring is chosen to be smaller, then there is a risk that its adjustment force is not sufficient to move the camshaft into the required end position in an emergency. Moreover, with this embodiment too, the camshaft can be moved out of its emergency running position in an uncontrolled manner in the event of particularly high strongly alternating torques.
Finally, DE 10 2004 033 522 A1 discloses a cam adjuster with electric drive, in which the adjustment device for adjusting the relative angular position of the camshaft with respect to the crankshaft has an adjustment drive as primary adjustment device and an auxiliary drive as secondary adjustment device. If the adjustment motor fails, the camshaft can be moved into a fixed angular position, the so-called emergency running position, by means of the auxiliary drive. A possible design of such an auxiliary drive includes a torsion spring which is pre-tensioned in a basic position by a displacement of the angle of rotation between the drive gear of the camshaft and part of the adjustment gearbox which is fixed to the camshaft and, in the event of a failure of the adjustment motor, effects a resetting by releasing the tension.
The disadvantage of this embodiment, however, is the costs associated with providing such an auxiliary drive. Further, the auxiliary drive requires a not insignificantly large proportion of the available installation space, which can likewise be seen as a disadvantage.